1. Field of the Invention
This invention relates to an engine knock control system which detects engine knock occurrence and controls the ignition timing or the like to prevent engine knock. Further, this invention relates to an engine knock detecting system for detecting engine knock occurrence.
2. Description of the Prior Art
There has been known an engine knock control system which, when engine knock occurrence is detected, controls a factor governing combustion in the engine such as the ignition timing to suppress the engine knock. For example, in Japanese Unexamined Patent Publication No. 58(1983)-28646, output of a knocking sensor during nonknocking operation is averaged, a reference level is determined on the basis of the averaged output level of the knocking sensor, and when the output level of the knocking sensor exceeds the reference level, it is determined that engine knock occurs. The output of the knocking sensor during nonknocking operation bears noise normally generated by operation of the engine including, for instance, noise generated by opening and closure of the valves, and accordingly, when knocking occurs, the output of the knocking sensor comes to bear oscillation due to the knock (knock signal) superposed on such background noise. Therefore, the reference level must be accurately set between the output level of the sensor solely bearing the background noise and that bearing both the background noise and the knock signal. Otherwise knock detecting accuracy is deteriorated to adversely affect knock control.
However, both the background noise and the knock signal vary from engine to engine, and differ according to change of the engine with time, the engine speed, the environmental temperature and the like. Further, they are affected by detecting accuracy of a given knocking sensor, change of the knocking sensor with time, and the like. Accordingly, when the reference level is set to a constant level, the background noise can be mistaken for the knock signal or knock occurrence cannot be precisely detected, whereby the engine output can be unnecessarily suppressed or the engine knock cannot be suppressed to adversely affect the engine durability.